/*
 *   This program is free software: you can redistribute it and/or modify
 *   it under the terms of the GNU General Public License as published by
 *   the Free Software Foundation, either version 3 of the License, or
 *   (at your option) any later version.
 *
 *   This program is distributed in the hope that it will be useful,
 *   but WITHOUT ANY WARRANTY; without even the implied warranty of
 *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *   GNU General Public License for more details.
 *
 *   You should have received a copy of the GNU General Public License
 *   along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

/*
 *    SpecialFunctions.java
 *    Copyright (C) 1999-2012 University of Waikato, Hamilton, New Zealand
 *
 */

package weka.core;

/**
 * Class implementing some mathematical functions.
 *
 * @author Eibe Frank (eibe@cs.waikato.ac.nz)
 * @version $Revision$
 */
public final class SpecialFunctions {

    /** Some constants */
    private static double log2 = Math.log(2);

    /**
     * Returns natural logarithm of factorial using gamma function.
     *
     * @param x the value
     * @return natural logarithm of factorial
     */
    public static double lnFactorial(double x) {

        return Statistics.lnGamma(x + 1);
    }

    /**
     * Returns base 2 logarithm of binomial coefficient using gamma function.
     *
     * @param a upper part of binomial coefficient
     * @param b lower part
     * @return the base 2 logarithm of the binominal coefficient a over b
     */
    public static double log2Binomial(double a, double b) {

        if (b > a) {
            throw new ArithmeticException("Can't compute binomial coefficient.");
        }
        return (lnFactorial(a) - lnFactorial(b) - lnFactorial(a - b)) / log2;
    }

    /**
     * Returns base 2 logarithm of multinomial using gamma function.
     *
     * @param a  upper part of multinomial coefficient
     * @param bs lower part
     * @return multinomial coefficient of a over the bs
     */
    public static double log2Multinomial(double a, double[] bs) {

        double sum = 0;
        int i;

        for (i = 0; i < bs.length; i++) {
            if (bs[i] > a) {
                throw new ArithmeticException("Can't compute multinomial coefficient.");
            } else {
                sum = sum + lnFactorial(bs[i]);
            }
        }
        return (lnFactorial(a) - sum) / log2;
    }

    /**
     * Main method for testing this class.
     */
    public static void main(String[] ops) {

        double[] doubles = { 1, 2, 3 };

        System.out.println("6!: " + Math.exp(SpecialFunctions.lnFactorial(6)));
        System.out.println("Binomial 6 over 2: " + Math.pow(2, SpecialFunctions.log2Binomial(6, 2)));
        System.out.println("Multinomial 6 over 1, 2, 3: " + Math.pow(2, SpecialFunctions.log2Multinomial(6, doubles)));
    }
}
